A new update will be available this week and will introduce Hoenn Pokémon in the game. At first, only 50 of the 135 Hoenn Pokémon will be available at launch.
The game is also introducing a dynamic weather system that will affect how Trainers discover, catch, and interact with Pokémon. Moves will also have increased or decreased effects during the weather just like the main games.
Weather Chart:
Clear: get stronger and appear more frequently
Rainy: get stronger and appear more frequently
Snowy: get stronger and appear more frequentlyWindy: get stronger and appear more frequentlyFoggy: get stronger and appear more frequently

Big Changes Are Coming to Pokémon GO!Pokémon from the Hoenn region and new weather effects will expand your Pokémon GO adventure.
Get ready to do a lot more exploring when Pokémon GO introduces a huge update that will expand your Pokédex and change the way you play! Dozens of new Pokémon originally discovered in the Hoenn region will soon start appearing for the first time. You'll be able to catch Treecko, Torchic, Mudkip, and many others, with even more Pokémon waiting to be discovered in the next several weeks.

Pokémon from the Hoenn region aren't the only new addition to Pokémon GO. The game is also introducing a dynamic weather system that will affect how Trainers discover, catch, and interact with Pokémon! The in-game map will be updated with cool new weather visuals based on your local weather patterns. The real-world weather will also influence which Pokémon you're likely to discover. For example, Water-type Pokémon will be more plentiful during rainy weather.

Likewise, attack types will be affected by certain weather conditions. A Charizard with Fire Spin, for example, will attack with greater heat when battling on a sunny day. Now, rain or shine, you're bound to have new—and even more exciting—Pokémon GO adventures!

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Starting later this week, Treecko, Torchic, Mudkip, and several more Pokémon originally discovered in the Hoenn region in the Pokémon Ruby and Pokémon Sapphire video games will start appearing in Pokémon GO for the first time ever—with even more to be discovered over the next several weeks! We’ll also be introducing a new dynamic weather system in Pokémon GO that will change the way you and the millions of Trainers around the world discover, catch, and interact with Pokémon.

You’ll want to start paying even closer attention to your local weather patterns, Trainers. In addition to the in-game map being updated with cool new weather visuals, weather near you will impact Pokémon in a variety of ways. For example, you’ll find your odds of discovering Mudkip splashing around will greatly improve on rainy days, as certain Pokémon types will be more commonly found during various weather conditions. Additionally, attacks of certain types will be more effective during some weather conditions, so a Charizard’s Fire Spin will perform better on sunny days, for example. Rain or shine, winter or spring, your gameplay experience will remain fresh, making your time outdoors even more exciting.
There’s never been a better time to grab your umbrella or snow boots and safely venture outside with family and friends this season. As you explore your local neighborhoods, remember to share a photo of your encounters on social media with the hashtag #PokemonGO. We'd love to see all of your new adventures as they unfold.